An active matrix liquid-crystal display (AMLCD) where thin film field-effect transistor (TFT) is used as a switching element of pixel has a high image quality, and is wide used as a display device of portable computer or recently a monitor for space-saving type desktop computer.
Recently, for the purpose of enhancing the view angle characteristic, a display system using the lateral electric field is proposed. This system is, for example, disclosed in Japanese patent application laid-open No. 6-160878 (1994). In this system, a pixel electrode and an opposing electrode are formed parallel on a same substrate, a voltage is applied between them to form parallel field on the surface of the substrate, thereby the direction of anisotropic axis (liquid-crystal director) of liquid crystal (hereinafter also referred to as `LC`) is varied, thereby the amount of transmitted light is controlled.
In this LC display system, where LC director moves only approximately parallel to the in-plane of the substrate, there does not occur the problem that, as in case of TN mode, the relationship between the amount of transmitted light and applied voltage is greatly changed between when viewing in the direction of LC director and when viewing in the direction of normal line of LC layer. Thus, it has the characteristic that nearly similar images can be obtained in a very wide view angle.
However, in this display mode, there are the problems that the response speed is lower than that of other modes and an image in movie display is not so good, because the twist transformation of LC mainly occurs therein.
Also, as shown in FIG. 1, a method where LC with negative permittivity anisotropy is vertically oriented and switching of pixel is conducted by applying a voltage between transparent electrodes 18 formed sandwiching LC so as to incline LC director 7 is proposed (e.g., 1997 SID International Symposium Digest of Technical Papers, p.845).
Meanwhile, in FIG. 1, 7 is a LC director, 8 is an emission-side polarizing plate, 10 is a transparent insulative substrate, 11 is orientation film, 12 is an incidence-side polarizing plate, 18 is transparent conductive film, and 21 is a dividing boundary.
In using such vertical orientation, though it is known that the response speed of LC is enhanced, there is the problem that the inversion of tone occurs when viewing obliquely in the inclined direction of LC in the case that the entire surface of LC is uniformly oriented and is inclined in one direction. Therefore, in using this method to get the wide view angle, it is necessary to divide the inclining direction of LC into several directions to be determined within one pixel.
Thus, to divide the inclining direction of LC within pixel by vertical field, it is necessary to form orientation state where LC is slightly inclined in several directions determined in the initial orientation state. Thereby, when applying vertical field, LC director can be inclined with more amount of inclination, therefore controlling the inclination direction into several directions.
As means for achieving this, for example, a method where the rubbing direction of the orientation film 11 is varied within pixel is proposed. However, it generally causes an increase in process. Also, LC materials with negative permittivity anisotropy necessary to compose this are limited, therefore the reliability and the working temperature range are questionable.
As a method for solving the problem of vertical orientation, Japanese patent publication No. 59-7367 (1984) discloses the method that, as shown in FIG. 2, LC with positive permittivity anisotropy is vertically oriented and LC director 7 is inclined by applying lateral field through comb-like electrode 24 formed on a transparent insulative substrate 10. Also in this method, for the purpose of determining uniquely the inclining direction of LC, it is necessary to incline, in advance, the orientation direction of LC to either side, because there exist two inclining directions of LC when LC is completely vertically oriented.
Also, as is the case with the prior art described earlier, it is necessary to divide the inclining direction within pixel to enlarge the view angle, because the view angle characteristic is not good in case of only one LC inclining direction. Thus, as is the case with the prior art described earlier, it is necessary to divide the initial orientation direction within pixel. This complicates the process of making LCD and causes an increase in manufacturing cost.
Also, Japanese patent application laid-open No. 56-88179 (1981) discloses a method where comb-like electrodes 24 are formed on both sides of a transparent insulative substrate 10 as shown in FIG. 3. In this method, the direction of electric field occurring when a voltage is applied to the comb-like electrode 24 is oblique. Thus, the oblique electric field 22 generates in two directions and therefore LC oriented vertically is inclined in the two directions. However, by reason that two kinds of electrodes are formed two transparent insulative substrates 10, the precision of overlapping is worse than that of the case that both are formed on a same substrate. Therefore, it is difficult to reduce the dispersion of electrode intervals and high-precision LC cell cannot be produced.
As described above, though some trials for producing high-speed LCD by using the quick response of vertical orientation have been already made, they have the problems, i.e., LC with negative permittivity anisotropy presents poor reliability of material, the complicated process such as orientation division is required, the electrode cannot be formed precisely, etc.